Pie menu and limited viewport shading options (show subset of an enum)
nope, to display a subset of an enum you can use wm.context_set_enum

        f = pie.operator("wm.context_set_enum", icon='SOLID', text='Solid')
        f.data_path='space_data.viewport_shade'
        f.value = 'SOLID'

        f = pie.operator("wm.context_set_enum", icon='WIRE', text='Wires')
        f.data_path='space_data.viewport_shade'
        f.value = 'WIREFRAME'




You might use a loop to reuse some code:

        modes = [
            ["SOLID", "SOLID", "Solid"],
            ["WIRE", "WIREFRAME", "Wires"],
            ["BBOX", "BOUNDBOX", "Bounds"]
        ]

        for ICON, VALUE, TEXT in modes:
            f = pie.operator("wm.context_set_enum", icon=ICON, text=TEXT)
            f.data_path='space_data.viewport_shade'
            f.value = VALUE